The present invention relates to a manufacturing technique for semiconductor devices and, particularly, to a technique effectively applied to a semiconductor module, in which a semiconductor element and a passive component are connected to a printed board by an intermetallic compound containing “no lead” (hereinafter referred to as “Pb-free”), and to a manufacturing method of the same.
Regarding conventional semiconductor modules, the following techniques have been considered by the inventors.
FIGS. 1 and 2 show sectional views semiconductor modules in the form of conventional structures. A semiconductor element 3 and a passive element 4 are connected onto a module board 6. After wire-bonding by wires 2, the module board is sealed with a mold resin 1 or inert gas 11 in a sealing casing 1. Pads 7 for the semiconductor module are reflow-soldered to pads 7 for the printed board 9 by medium-temperature Pb-free solder 8 of Sn—Ag—Cu base. The melting point of the Sn—Ag—Cu base Pb-free solder is as high as approximately 220° C., and it is assumed that the solder is heated up to 260° C. at maximum at a time of reflow connection. Therefore, for the purpose of temperature hierarchy, high-lead solder with a melting point of 290° C. or higher is used for connection of the semiconductor element 3 and the passive element 4 inside the semiconductor module. For this reason, high refractory ceramic boards (for example, Al2O3, AlN, or Si3N4) or metal boards (of, for example, Al, Cu, or Fe base) are used as the module boards 6 of the semiconductor module. However, use of such module boards 6 results in high cost and further manufacture of semiconductor packages results in high cost. Moreover, the ceramic or metal-module board is generally heavy and thick, and this results in increases in weight and back height of parts.
A board capable of lowering in cost, lightening in weight, thinning, and decreasing in package back height includes a printed board such as FR-4 for use in mounting the semiconductor element or passive element by the Sn—Ag—Cu base solder. However, such a board has a heat-resistant temperature of 300° C. or lower, so that the connection by high-melting solder such as high-lead solder or Au-20Sn causes damage to the board by heating, which results in destruction of the board. To use the printed board as the module board, it is desirable that internal connection is made by a material capable of connecting at a temperature of 260° C. or lower for secondary packaging. However, if the internal connection is made by solder with a melting point of 260° C. or lower, the solder is remelted at the time of reflow soldering. When a circumference of a connection portion is molded with a resin and the solder inside thereof is remelted, as shown in FIG. 3, there often arises what is called “flash”, i.e., a leakage of the solder 8 from an interface between the mold resin 1 and the module board 6 due to-volume expansion by melting. Even if not leaked, the solder acts to leak therefrom and, as a result, large voids 12 are formed in the solder after coagulation and the printed boards are defective products.
For example, by non-patent document 1 (“High Temperature Joints Manufactured at Low Temperature” written by William W. So, et al., Proceeding of ECTC, 1998, p. 284), the following has been reported. GaAs having a back surface metallized with Cr: 0.03 μm/Sn: 2.5 μm/Cu: 0.1 μm and a board (Glass) metallized with Cr: 0.03 μm/Cu: 4.4 μm/Au: 0.1 μm are connected to each other at 280° C. and then are kept for 16 to 24 hours, so that the melting point of the connection portion can be made high by transforming completely the connection portion into Cu3Sn. Similarly, Si having a back surface metallized with Cr: 0.03 μm/In: 3.0 μm/Ag: 0.5 μm and Si metallized with Cr: 0.03 μm/Au: 0.05 μm/Ag: 5.5 μm/Au: 0.05 μm are connected to each other at 160 to 200° C. and then are subject to a aging treatment for 16 to 24 hours at 150° C., so that the melting point of the connection portion can made high by transforming the connection portion into “an Ag-rich alloy+Ag3In”.
Also, in non-patent document 2 (“Study on transforming a micro-connection portion into an intermetallic compound by using Sn—Ag solder” written by Yamamoto, et al., Summary of MES 2003, Oct. 2003, p. 45), the following has been reported. Ni-xCo (x=0.10) metallized with Sn-3.5Ag: 26 μm; solder; and Kovar metallized with Ni-20Co: 5 μm/Au: 1 μm are connected together at 240° C. and then are kept for 30 minutes, so that all the connection portions can be transformed into (Ni, Co) Sn2+(Ni, Co)3Sn4 and their melting points can be made high. By the use of Ni-20Co containing Co for metallization, a growth rate of the compound can be increased.
By using the above-described schemes, the connection can be made at a temperature of 260° C. or lower, so that once the connection portion completely becomes a high-melting layer, the connection portion is not remelted and can be maintained even if being heated to 260° C. at the time of reflow soldering.